Any modulation method can be represented by a constellation. An example of this is the eight point constellation illustrated in FIG. 3. This constellation is generated from differentially encoded QPSK (DEQPSK or .pi./4 QPSK) which is a subset of the four state QPSK constellation. Each state is characterized by a vector having the same magnitude, but a different phase angle. Due to the differential encoding of the QPSK signal to generate the .pi./4-QPSK signal, data recovery may be accomplished with either a coherent or non-coherent detector. Coherent detection exhibits better performance in some situations as compared to non-coherent detection techniques.
In coherent detection, however, the carrier phase must be recovered from the received signal. Also, with .pi./4-QPSK a known initial 0.degree./45.degree. rotation phase is required as a start-up condition. This is required to determine the initial constellation point of the QPSK constellation. Additionally, environmental fading can rotate the constellation points causing random phase modulation. There is a resulting need for a coherent detector that can determine the initial constellation point rapidly and track the rotating constellation.